fix: regenerate icons as square PNGs for AppImage bundler

Source icon was 979x896 (non-square). ImageMagick preserved aspect ratio
when resizing, producing non-square outputs (e.g. 512x469). The AppImage
bundler panics if no square icon is found.

Fix: pad source to 979x979 with transparent background before resizing.
All icon sizes are now perfectly square: 32x32, 128x128, 256x256, 512x512.

Co-Authored-By: Claude Sonnet 4.6 <[email protected]>